ESTABLISHING BIOGENICITY CRITERIA FOR ARCHEAN ENDOLITHS

Rock boring microbes known as endoliths may have been well adapted to the harsh surface environments on early earth. Fossilised endolithic microborings are known from c.1.5 Ga stromatolitic carbonates (1), and have recently been reported from 3.48 Ga pillow basalts (2). Assessing the biogenicity of such endolithic microborings requires a novel set of biogenicity criteria. To formulate these we use our investigations of a new assemblage of microtubular structures in a ~3.5 Ga sediment from Western Australia:

An Archean Age – endoliths may colonise a rock substrate at many points in geological time and are notorious contaminants. We report the results of detailed light microscopy and SEM work that allow us to identify multiple generations of the Australian microtubes within a depositional and diagenetic framework.

Biogenic morphology – endoliths known in sediments produce tubular cavities that often taper and may show simple branching. Endoliths described from modern volcanic glasses produce more varied morphologies: curved and coiled microtubes with smooth or scalloped edges and sometimes granular textures (3). Our Australian microtubes include two morphotypes: type A tubes that are straight with a modal width of 5µm that often occur in bundles; type B tubes which are curved and tangled together with more variable widths. We posit that these correspond to more than two mechanisms of formation and include examples of ambient inclusion trails formed by the migration of mineral inclusions.

Geochemical evidence for biological processing – microborings are trace rather than body fossils that may be infilled by younger phases. High spatial resolution is required to search for the decayed remains of the endoliths or linings they may have left on the tube walls. Demonstrating the syngenicity of these features can be difficult, and perhaps a more robust bio-indicator is the reported depletion of elements in the matrix surrounding microborings in volcanic glass. To provide this we describe the results of FIB-TEM and nanoSIMS mapping of finescale (~100nm) elemental (C, S, N, O, P, Fe) and isotopic variations associated with the Australian microtubes.

1 Zhang Y (1987). Acta Micropaleontoligica Sinica 4, 1. 2 Furnes H.R., et al. (2004). Science 304, 578. 3 Banerjee N.R., Muehlenbachs K. (2003). G3. 4, 1037.